1. Field of the Invention
The present invention relates to a polymer clad optical fiber comprising a core made of quartz glass or optical glass and a cladding made of a polymer. More particularly, the present invention relates to a polymer clad optical fiber comprising a cladding made of a ladder type polymethylsiloxane.
2. Description of the Related Art
As a cladding polymer of the conventional polymer clad optical fiber (hereinafter referred to as "PCF"), are used a silicone resin (cf. Japanese Patent Publication No. 2321/1981), a fluoroalkyl methacrylate polymer (cf. Japanese Patent Kokai Publication No. 12146/1983), a copolymer of vinylidene fluoride and tetrafluoroethylene (cf. Japanese Patent Publication No. 41966/1981), polyetheresteramide (cf. Japanese Patent Kokai Publication No. 60402/1981), and a UV light curable fluorinated acrylate composition (cf. U.S. Pat. No. 4,211,209).
These cladding polymers cannot satisfy high level requirements for the polymer clad optical fiber such as decrease of light transmission loss, easy fitting of a caulking type connector to the optical fiber, stability of temperature characteristics of light transmission loss and the like.
For example, since the silicone resin has poor mechanical characteristics, in particular, mechanical strength, the light transmission loss of a PCF comprising the silicone resin as the cladding polymer is increased when the caulking type connector is used.
Although the fluoroalkyl methacrylate polymer is highly transparent, its adhesivity to the core glass is insufficient.
Since the copolymer of vinylidene fluoride and tetrafluoroethylene and the polyetheresteramide have large scattering and absorption, they have inferior light transmission, and the light transmission loss of the optical fiber cannot be decreased.
Since the UV light curable fluorinated acrylate composition is cured with UV light irradiation after it is applied on the core glass, it is difficult to control the degree of curing, and residual stress caused by shrinkage of the polymer during curing increases the transmission loss of the optical fiber. In addition, adjustment of an outer diameter of the optical fiber is difficult.
As a cladding material which satisfies the above described requirements, a ladder type polysiloxane (organosilsesquioxane) is proposed (cf. U.S. Pat. No. 4,835,057).
However, a cured material of the ladder type polysiloxane has very small elongation. When it is coated on the glass core as the cladding material, its surface may be cracked because of the presence of bending strain, strain generated by difference of coefficients of thermal expansion between the core glass and the ladder type polysiloxane, or strain due to shrinkage of the ladder type polysiloxane during curing. Since the ladder type polysiloxane is in general dissolved or dispersed in a solvent, coated on the core glass and then cured by heating simultaneously with evaporation of the solvent, bubbles generated from the solvent may be trapped in the cured polysiloxane and increase the light transmission loss.
In addition, the ladder type polymethylsiloxane is degraded since the side methyl groups are cleaved and dissociated by oxidation in a high temperature atmosphere.
The above drawbacks of the ladder type polysiloxane are not overcome by the invention disclosed in U.S. Pat. No. 4,835,057.